The present invention relates to a temperature responsive safety switch, fuse, or circuit breaker, of the type having a meltable insert melting of which opens the switch.
Temperature responsive safety switches including a meltable solder insert are known in general and are disclosed, for example in German Auslegeschrift [Published Patent Application] No. 2,012,426. In another prior art temperature responsive safety switch of the type under consideration, the meltable solder insert comprises a meltable solder in the form of a round disc or wafer which is fastened in a recess of a housing of insulating material and bridges an opening in a heat transfer plate made of a material having good thermal conductivity. The diameter of this opening is larger than the diameter of an actuating plunger which rests on the upper side of the disc. When there is enough heat to cause the solder to melt such melting has the result of eliminating the cover over the opening, since that cover was constituted by the solder disc. Due to the force of the contact spring and simultaneous interruption of the electric current path, the actuating plunger is then pressed into the opening in the heat transfer plate.
The drawback of this prior art temperature responsive safety switch is its poor service life. This has a disadvantageous effect on its use as a temperature responsive fuse. Although the particular alloy composition of the melting solders employed leads to the expectation that a defined melting point exists, the solder will soften somewhat before such melting temperature is reached. That means that if there is a long duration heating influence, but at a temperature below the intended actuation temperature, the melting solder will lose firmness, as a result of which the force exerted by the contact spring on the actuating plunger may lead to premature opening of the temperature responsive fuse. Moreover that the melting solder is essentially exposed to the atmosphere which is a drawback since the presence of oxygen may change the melting point of some solders.